A computer system is made of software, data, hardware, communications and people; each computer system ca be divided into a set of sub systems. Each sub system can be divided into another sub system until each sub system performs a single task.
The division of sub system is by “top down design”. The process of breaking down into smaller sub systems is called ‘step wise refinement’.
Structure diagrams can be used, it shows the design of a computer system in a hierarchical way, with each level giving a more detailed breakdown of the system into sub systems.
A Sub-Routine is set of programming instructions for a given task that forms a sub-system, not the whole system. Sub-Routines written in high level languages are called ‘procedures’ or ‘functions’ depending on how they are used.
Validation is the automated checking by a program that data is reasonable and in the right criteria that it is accepted into a computer system. There are many different kinds of data check such as: range checks, length checks, type checks, character checks, presence checks, etc. When data is validated by a computer system, if the data is rejected a message should be output explaining why the data was rejected and a second opportunity given to enter the correct data.